As Barack Obama's chief of staff, Rahm Emanuel is arguably the second most powerful man in America. It's a job tailor-made for the unflappableóbut that's not a description anyone would ever apply to Emanuel, not even when he was a political rookie raising money for Chicago Mayor Richard Daley.

If donors didn't agree to quintuple their normal contributions, Emanuel scolded them for their stinginess and hung up the phone.

Obama senior adviser David Axelrod, who has known Emanuel for decades, contends the method displayed a level of chutzpah that "redefined the term." And it worked. Emanuel broke all fund-raising records.

Two things are absolutely clear about Rahm Emanuel. He is effective and he is extreme.

Obama himself regularly jokes about Emanuel's profanity: "For Rahm, every day is a swearing-in ceremony."

These days, Emanuel is making deliberate efforts to tone himself down. "I'm not yelling at people; I'm not jumping on tables [anymore]," he told Mark Leibovich of The New York Times.

Still, Emanuel displays many characteristics of a hypomanic temperament. This mildly manic dispositionówhich is not a mental illnessócomes with assets that could propel someone to the top of his field:

immense energy, drive, confidence, creativity, and infectious enthusiasm. I have found through interviews and historical accounts that hypomania has animated many leaders, from Alexander Hamilton and Andrew Carnegie to Emanuel's former boss Bill Clinton.

But it also carries a cluster of liabilities: overconfidence, irritability, and especially impulsivity that often pitches the hypomanic into hostility.

Drives are heightened and impulse control is weakened, making the hypomanic brain like a Porsche with no brakes. In keeping with his hypomanic temperament, Emanuel doesn't need much sleep and he can't stay still.

"He's like a shark that always has to keep moving or he dies," says John Lapp, who worked for Emanuel.